[Boatanchors] Burying Coax

[Jim Wiley]

Keith -


When using high quality, properly terminated  coaxial cable, signals on 
the "outside" of the shield do not transfer to the "inside" of the 
shield.  Only when coax is improperly terminated, or has a shield braid 
that "leaks" RF (less than 100% shielding) will signals transfer from 
the outside to the inside of the shield, or vice versa. 


Note that I am speaking only about the shield.  The center conductor of 
course carries the desired signal, but again, in a properly terminated 
cable the signals traveling up or down the cable do not appear on the 
"outside" of the shield.    For this reason, it should be apparent that 
noise outside the cable will not propagate to the inside of the cable, 
buried or not.  Please note, there is the caveat that there must be no 
direct (RF) connection  between the inside and outside of the cable at 
either end.   Even though there appears to be such a connection (an 
ohmmeter will show zero ohms between the inside and outside of the 
shield),  the inside is not the same for RF. 


Keeping the inside and outside separate (for RF) takes some care.  One 
way to accomplish this is to use ferrite beads or cores over the outside 
of the cable.  The ferrite forms an effective RF  choke that  keeps the 
signals separate.   The fact that there is a "DC" connection between the 
inside and the outside of the cable is not relevant.  


The fact that the shield braid is slightly leaky, even on the best 
cables, is one reason why you see cables with both a braid and a foil 
shield.    The purpose of the foil barrier is to provide a 100% shield.  
Unfortunately, foil is not very strong, and is difficult to connect to, 
electrically speaking (aluminum foil will not "take" solder easily).  
For this reason, and because the  braided both protects the foil and 
adds mechanical strength, as well as providing for a solder friendly 
surface, these cables often have both types of shield.


Of course, the best cables for shielding are those that have a solid, 
impermeable outer shield.  Hardlines such as Andrew Corp Heliax (r) are 
an example of this type of cable.  unfortunately, unless you are a good 
"horse-trader" or have access to commercial resources,  this type of 
cable is usually prohibitively expensive for ham use.


- Jim, KL7CC




Keith Densmore wrote:
> Thanks to all you great guys with the information on the ditch witch. Sounds 
> like that will do it. Glad there is a machine for that, hi.
>
> But a couple of you mentioned that burying coax will not lower noise pickup. 
> Which has caused me to put this plan on hold. I need to know more.
>
> I 'thought' coax to be a good insulator of noise but not perfect. My belief 
> is or was that the outer shield forms a capacitor with the inner conductor 
> and therefore a noise signal on the outer braid will to some extent 
> capacitivily couple itself into the center conductor. In my case the antenna 
> is in a low noise location but the coax runs through a noisier area. I 
> listen to 40 meters down to VLF, VLF being very prone to AC line noises.
>
> Anyone know for sure?
>
> Thanks, 73,
> Keith
>
>
> ______________________________________________________________
> Boatanchors mailing list
> Home: http://mailman.qth.net/mailman/listinfo/boatanchors
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Boatanchors at mailman.qth.net
>
> List Administrator: Duane Fischer, W8DBF
> ** For Assistance: dfischer at usol.com ** 
>
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>